The benefits of Communion/Sabbath for Masters are:

2 Slaves +1 to all Magic Paths, Including Holy/Unholy
4 Slaves +2 to all Magic Paths, Including Holy/Unholy
8 Slaves +3 to all Magic Paths, Including Holy/Unholy
16 Slaves +4 to all Magic Paths, Including Holy/Unholy

And so on and so on. Then of course fatigue is distributed among the slaves by the masters of the sabbath/communion.

You can only have a Crystal item work on someone who has at least 1 path in magic.

There is no benefit of having odd numbers of slaves except for Fatigue distribution.